The invention relates to an image display method and apparatus using a liquid crystal display unit which has a memory function and is matrix driven and, more particularly, to an image display method and apparatus displaying and driving a liquid crystal display unit of a phase transition type at a high speed.
In association with the recent progress of the realization of the office automation, a liquid crystal projector apparatus using a liquid crystal display (LCD) unit has been developed. By sending a CRT picture plane of a personal computer to the projector apparatus and projecting and displaying it, such a picture plane is used for presentation or the like. Hitherto, as a liquid crystal display unit, a TFT liquid crystal display unit of an active matrix driving type is known. Although such a display unit can also display a motion image by a high speed driving at a video rate, a pixel density is as low as, for example, up to (1280 lines.times.1024 pixels) at the A4 size. According to the TFT liquid crystal display unit, although a rewriting operation of a picture plane can be performed at a high speed, a transmittance of the light is low and the screen is dark. As compared with the liquid crystal display unit of the active matrix driving type such as a TFT liquid crystal or the like, a liquid crystal display unit of a simple matrix driving type has a simple structure and is suitable for realization of a large number of pixels and high precision. There is a liquid crystal display unit of a phase transition type as one of the liquid crystal display units of the simple matrix driving type. At present, the phase transition type LCD unit of the A4 size having (2500 lines.times.3500 pixels) or more has been put into practical use. The phase transition type LCD unit has a structure such that transparent line electrodes and data electrodes are arranged in a matrix form, and a liquid crystal material is sandwiched between the electrodes at an intersecting position of both of the line and data electrodes. To voltage drive the matrix-shaped electrodes, two driver circuits of a line electrode driver and a data electrode driver are used. In a display operation of the phase transition type LCD unit, a line electrode to be displayed is selected and a voltage is applied thereto by the line electrode driver. The data electrode driver applies a voltage according to a pixel to be displayed and a pixel not to be displayed in pixel data to be written. When no voltage is applied, the liquid crystal of each pixel on the selected line electrode becomes a cholesteric phase by which the incident light is scattered, so that "black" of a low light transmittance is displayed. On the contrary, when a voltage is applied, the liquid crystal changes to a nematic phase having a high light transmittance and transparent "white" is displayed. As mentioned above, a display is executed due to a difference between the light transmittances. By sequentially repeating similar processes with respect to all lines, the display of the whole screen is accomplished. The phase transition type LCD unit has a memory function and display data which has once been written is displayed as it is so long as a holding voltage is applied.
The details of the phase transition type LCD unit have been disclosed in, for example,
Phase transition type projection display (Display and Imaging, Vol. 1, No. 1, pp. 61-69, 1992) PA1 A5-M pixel overhead projection display utilizing a nematic-cholesteric phase-transition liquid crystal (Journal of the Society for Information Display, Vol. 1, No. 1, pp. 43-49, 1993) PA1 (the order of display lines to be written and displayed is set). PA1 (line data is read out from an image data storing unit in accordance with the order set in the display line setting step). PA1 (each time the line data is read out in the reading step, the line data and each line data in the image data storing unit are compared, thereby detecting the write display line for the display unit). PA1 (the line data read out in the reading step is written and displayed to the display lines of one or a plurality of display units including the present processing line detected in the write line detecting step). PA1 (the line data is read out from the image data of the image data storing unit in accordance with a predetermined order every display area of the display unit and the line data is compared with each line data of the same display area in an image data storing unit, thereby detecting one or a plurality of write lines for the display unit). PA1 (the line data read out at present is written and displayed to the write line of every display area detected in the write line detecting step). PA1 (the line data of a plurality of lines is read out from the image data in the image data storing unit every display area of the display unit and common line data included commonly in the plurality of lines and the write lines are detected). PA1 (the corresponding common line data is simultaneously written and displayed to the write lines of every display area detected in the common line data detecting step).
According to the phase transition type LCD unit as mentioned above, although the resolution is high, the light transmittance is high and the screen is bright, there are problems such that the display speed is slow and the rewriting time of the picture plane is slower than that of the TFT LCD unit. Namely, it takes tens of milliseconds to rewrite from black (scattering state) to white (transparent state) and, contrarily, a writing time of a few milliseconds is required to rewrite from white (transparent state) to black (scattering state). For example, it takes about 20 seconds to display an image after the phase transition type LCD unit of (2500 lines.times.3500 pixels) was initialized. To solve the above problem such that the rewriting time of the picture plane of the phase transition type LCD unit is slow, in the conventional display control, data of a previous picture plane and data of a new picture plane are always compared and only a different portion is rewritten, thereby reducing the rewriting time (refer to JP-A-61-138991, JP-A-2-217893, JP-A-7-5845, etc.). In the conventional display control in which only the different portion between the new and previous picture planes is rewritten, there is an effect when a picture plane having continuous contents like a motion image is switched. In a liquid crystal projector for mainly displaying a still image, however, there are many cases where the contents of the picture plane are exactly different every time. Even if only the portion different from the previous picture plane is rewritten, a reduction of the rewriting time cannot be expected.
When switching the picture plane of the phase transition type LCD unit, for example, since 2500 lines are sequentially displayed for an enough time of about ten seconds, the picture plane is slowly rewritten from the top to the bottom and it takes a time until the contents of the next picture plane are grasped. Therefore, in case of retrieving desired contents while switching the picture plane, it is troublesome to recognize the retrieval picture plane and retrieval takes a too long.
Further, in the phase transition type LCD unit, when an interlace writing to realize a high speed writing operation which is used in a CRT or the like is performed, since a whole writing speed is slow, if the writing operations are sequentially performed in accordance with the order of the odd line and the even line, there is a problem such that the contrast of the display during the writing operation deteriorates. Since the time required for writing and required for erasing are not equal, as in the CRT, the interlace writing such that data is divided into blocks on a plural line unit basis and the interlace writing is executed in accordance with the order of the odd blocks and even blocks cannot also be performed. Therefore, with respect to the phase transition type LCD unit, the display time using the interlace writing cannot be made fast.
When the phase transition type LCD unit of the matrix driving type is used in a liquid crystal projector apparatus or the like, the liquid crystal projector apparatus is connected as an external display apparatus to a personal computer, and image data formed by the personal computer is sequentially selected from the top line and is transferred and displayed. In this case, if the display speed of the phase transition type LCD unit can be made fast, a problem occurs such that a data transfer speed cannot follow a display speed and the display speed is, consequently, limited by the data transfer speed.